Resuscitators are used to supply breathing gas to a patient who may not be breathing spontaneously. Portable resuscitators may take the form of a resilient bag that is squeezed manually to supply a volume of air to the patient, the bag refilling with air when it is released so that a new volume of air can be supplied. Alternatively, the resuscitator may be a mechanical device including a tiring valve and various other controls and is connected to an oxygen cylinder, which both provides the breathing gas, or a part of this, and which may also provide the power to drive the components of the resuscitator. Examples of such resuscitators are described in GB 2174760, GB 2174609, EP 343818, EP 342883, EP 343824, GB 2282542, EP 691137, GB 2284159 and GB 2270629. These resuscitators are arranged to supply gas in a cyclic manner to the patient at a rate compatible with normal breathing. The resuscitator usually has some form of manual override so that gas can be provided at a selectively controllable rate such as when the patient is receiving CPR. Existing resuscitators suffer from various problems. For example, where the resuscitator can be operated fully manually, there is a risk that an inexperienced operator could provide inappropriate rates of breathing with possible danger to the patient. Other resuscitators do not allow sufficient flexibility in the administration of gas.